Method and apparatus for imparting an electrostatic charge to a layer of insulating material



Fb. 7, 1967 e. NAUMANN ETAL 3,303,401 ATIC US FOR IMPARTING AN ELECTROST LAYER OF INSULATING MATERIAL Filed July 17, 1963 CHARGE TO A METHOD AND APPARAT INVENTORS GERHARD NAUMANN ENDE RMANN FRITZ BY United States lateht @ilice 6303561. METHOD ANDAPPARATUS on IMPARTENGAN .ELECIROSTATJC CHARGE To A LAYER OF IN- SULATING MATERIAL Gerhard Naurnann and Fritz Ende'rmann, iWiesb-aden,

8 Claims. Ci. 317262) The invention relates to a method for imparting an electrostatic charge to a layer of photoconductive or other insulating material.

Most existing forms of charging apparatus consist of wires connected to a source of high voltage from which discharge currents are emitted. Thedischarge along wires is very uneven, which results in a correspondingly uneven charging of the layer, and this is intensified when the wires are connected to the negative pole of the highvoltage source. In the case, however, of electrophotography, it is necessary to impart a complete and uniform charge to a relatively large surface.

It has been proposed to increase the uniformity of charge by providing a wire grid between the discharge wires and the layer to be charged which is maintained at a definite potential. By its electrostatic protective action, the grid terminates charging after a certain time, i.e. when the charge on the layer corresponds to the potential of the grid. As the charge on the layer increases, the strength of the charging current accordingly decreases progressively and reaches zero when an end state has been reached. Also, the grid takes a considerable part of the discharge current directed towards the layer to be charged. Such apparatus has the disadvantage that the action of the grid in limiting the discharge current which is effective for charging is in direct proportion to its action in imparting uniformity to the charge and it is necessary to wait for the end state to achieve uniform charging. Uniform charging is therefore obtained only with the accompanying disadvantage of a slow rate of charging.

In another known charging apparatus, a single discharge wire is surrounded by a screen, having an opening facing the area to be charged and electrically connected to the support, which is generally grounded, for the insulating layer to be charged. The screen captures the greater part of the current of ions from the discharge wire and so enables the charging apparatus to be used at relatively high voltages. Accordingly, emission no longer takes place only from individual points of the wire, as it does without the screen, but is fairly uniformly distributed over the surface of the wire. In this case also, the effectiveness of the current decreases with increasing effectiveness of the screen and the rate of charging is also low.

A further disadvantage which is common to all fine discharge wires extending parallel to the layer being charged is a high susceptibility to mechanical trouble, which increases with the length of the discharge as these, when long, tend to vibrate with a considerable amplitude, so that their life is considerably reduced both as the result of mechanical strain and as the result of the spark discharge to which they are susceptible.

It has also been proposed to use a discharge device consisting of discharge points arranged on a fiat surface or on the surface of a rotating cylinder, in conjunction with auxiliary electrodes constituted by rods extending parallel to the surface of the layer to be charged, but this gives uneven charging because such rod-shaped aux- 363,461 Patented Feb. 7, 1967 iliary'elect rodes do not exert any appreciable iniluene onthe discharge current of the discharge electrode. p

The invention"'provid'es ainetho'd for elctrostatically charging a photdcdnducfive or other insulating layer which cdinprises exposing the layer to the discharge between a discharge electrode connected to one pole of a direct'current' source and xtnding in a direction approximately perpendicular to the surface of the layer to be charged, and a counter-electrode situated on the other side of the layer and connected to the other pole of the source, an auxiliary electrode spaced laterally from the discharge electrode and extending in a direc tion approximately perpendicular to the surface to be charged being also connected to the other pole of the source.

The invention also includes apparatus for electrostatically charging a photoconductive or other insulating layer and includes a discharge electrode which extends in a direction approximately perpendicular to the path traversed by the layer through the charging apparatus, a counter-electrode situated on the other side of the path, an auxiliary electrode adjacent to and spaced laterally from the discharge electrode and also extending in a direction approximately perpendicular to the surface to be charged, a source of DC. voltage and means for connecting one pole of the source to the discharge electrode and the other pole to the counter-electrode and to the auxiliary electrode or electrodes. 1

The invention permits of uniform charging of an insulating layer with any voltage of interest for practical purposes notwithstanding the fact that the layer is traversed at high speed and with a single pass through the charging apparatus.

The apparatus is sturdy and safe in operation, easy to handle and gives reproducible results. Unlike apparatus in which the discharge electrode is constituted by thin wires extending parallel to the layer being charged and which tend to vibrate resulting in irregular spacing from the surface being charged, it allows the charging of layers of considerable width. The slight extent of the spray area of the discharge electrode in the moving direction of the material to be charged, has the advantage of enabling even very flexible or corrugated layers to be charged, because the charging current need not act simultaneously across distances which dilfer widely.

The apparatus has the further advantage that it requires a single source of voltage, as opposed to the two or more sources of voltage required for apparatus using grids as auxiliary electrodes.

The invention will be further described with reference to the accompanying drawings in which:

FIGURE 1 is a schematic representation of the basic principle of the invention,

FiGURE 2 is a schematic view in front elevation of an apparatus utilizing the principle represented in FIGURE 1,

FIGURE 3 is a schematic representation of another embodiment of an apparatus in accordance with the invention, and

FIGURE 4 is a schematic representation of still another embodiment of the apparatus of the present invention.

Referring to FIGURE 1 of the drawings, the layer 4 to be charged has a grounded counter-electrode 3 mounted beneath it and mounted above it is the discharge electrode 1 having the auxiliary electrodes 2 and 2a positioned at either side thereof.

In FIGURE 2, a number of pointed discharge electrodes 1 are spaced across the width of the layer 3 and are connected by a metal bus bar 5. The auxiliary electrode 2a, shown in FIGURE 1, is omitted but the other auxiliary electrode is indicated at 2.

In the construction of FIGURE 3, the material 4 to be charged 'is fed by the feed rolls 7 and 7a over a roll 6, the latter constituting the counter-electrode, to the transpbrt rolls 7b and 7c. The discharge electrode 1 and the auxiliary electrodes 2 are shown in the same relative positions as in FIGURE 1.

In the embodiment of FIGURE 4, the two auxiliary electrodes are constituted by the rolls 8 and 9 which also act in conjunction with the rolls 8a and 9a as feed and transport rolls, respectively. This embodiment of the apparatus includes a counter-electrode in the form of the roll 6 and the discharge electrode 1.

A fundamental requirement of the discharge electrode is that it should extend approximately perpendicularly towards the surface to be charged. Preferably, the end of the discharge electrode facing the surface to :be charged is pointed or has a knife edge. When the discharge electrode is pointed, it can, for example, be in the form of a needle or wire or it may be triangular or conical. Particularly good results are obtained when the discharge electrode comprises a number of such needles disposed side by side in a line extending transversely to the path traversed by the material through the apparatus and connected together and to a voltage source at the ends remote from the material to be charged.

The member connecting the individual needles may be a metal rod, a metal strip, or a plastic member having a conductive core.

The discharge electrode may also, however, be constituted by a strip having a knife edge facing the material to be charged. The thickness of the strip is not important. The mechanical stability of such a strip is, of course, greater than that of a series of needles but it is nevertheless advantageous to support the strip in a mechanically stable mounting. The discharge electrode may also comprise a series of needles and knife-edged strips disposed side by side and alternating either regularly or irregularly.

The discharge electrode must, as noted above, extend in a direction approximately perpendicular to the material to be charged; Its length, measured in this direc-' tion, is not important and may vary from a fraction of a millimeter to 50 mm. and is preferably between 2 and 30 mm. In practice, a length of 20 to 25 mm. has proved satisfactory. When the discharge electrode is constituted by spaced needles, the distance between them can vary Within limits. It is. preferred that this distance, which is shown in FIGURE 2 as d bears the specific relationship noted below to the spacing of the auxiliary electrodes from the discharge electrode, shown as d in FIGURE 1, and the distance between the'extremity of the discharge electrode and the surface of the material to be charged, shown as d;, in FIGURE 1. a

The discharge electrode must be made of a material of high electrical conductivity, and exemplary are stainless steel, bronze and brass.

The auxiliary electrodes also extend in a direction approximately perpendicular to the material to be charged and they may have, as in FIGURE 2, an elongated fiat configuration, being made of ribbon or strip material. The thickness of the strip is not important. It may be one or more millimeters. The length of the auxiliary electrodes, measured in a direction perpendicular to the material to be charged, should be 10 mm. or more. The length of the auxiliary electrode transverse to the direction of advance of the material to be charged should be about the same as the length of the discharge electrode measured in the same direction. The auxiliary electrodes may be made of the same electrically conducting material as. the discharge electrode, or may be made of an insulating material having a conductive coating.

The spacing of the auxiliary electrodes from the material to be charged may be the same as, but is preferably less than, that of the discharge electrode. I

Either one or two auxiliary electrodes may be used.

When two auxiliary electrodes are used, it is not necessary for them both to have the same configuration.

Where, as in FIGURE 4, the auxiliary electrodes are constituted by rolls which also act to feed the material to be charged, the surface line of such a cylindrical auxiliary electrode which is nearest to the discharge electrode should be spaced from the material by approximately the same distance as the discharge electrode.

In all cases, no part of the auxiliary electrodes extends into the discharge space between the material being charged and the facing end of the discharge electrode.

The counter-electrode may be a plate or roll and more than one counter-electrode may be employed if desired.

The potential required for charging is derived from a DC. voltage source 10, one pole of which is connected to the discharge electrode and the other of which is connected to the auxiliary electrodes and to the counterelectrodes. This other pole is normally grounded.

The voltage utilized depends upon the level of charge to be imparted to the insulating layer, upon the configuration of the discharge electrode, and its spacing from the layer and from the auxiliary electrodes. A high current density is generally desirable and this, of course, requires a high voltage.

As mentioned above, the distances d d and d are important in the achievement of uniformity and a high rate of charging. In general, d should be greater than d and (1 should be greater than d The distance (1;, usually exceeds 10 mm. and is preferably between 13 and 18 mm. The distances d d and d are preferably in the ratios of 3:2:1 and, while these ratios may be varied, d and d should never be so small as to permit sparking.

The invention can be applied to the charging of insulating layers in general, such as plastic foils of thermosetting or thermoplastic materials, e.g., polyvinyl chloride, polyethylene and polyester foils. The invention can be used with particular advantage for the charging of photoconductive insulating layers; these may contain organic or inorganic photoconductors with or without binders, additives and activators, and may be carried on supports of paper or metal, e.g., aluminum foil.

As an alternative to using a grounded counter-electrode, the insulating layer may be sprayed from both sides independently with charges of opposite polarity. In this case, the counter-electrode is constituted by another discharge electrode connectedto the opposite pole of the voltage source.

Another advantage of the invention is that the intensity of the charging current, and therefore the level of charge, can be varied within wide limits. This is of importance because the levels of charge required vary with the thickness of the insulating layer. This variation can be effected in three ways: (1) by altering the spacing between the discharge electrode and the surface of the layer, since the current strength decreases with increase in spacing; (2) by varying the speed of traverse of the insulating layer, since the level of charge acquired by a flat insulatinglayer is inversely proportional to its speed of traverse, and (3) variation of the voltage.

The apparatus according to the invention permits of operation with a voltage which only just exceeds that necessary for a discharge current to flow as well as with i a considerably higher voltage. Variation of the conditions in these ways does not detrimentally affect the uniformity of charging.

Since the discharge electrode extends perpendicular to the layer being charged, it is not subject to objectionable vibration, which arises only in the case of thin electrodes extending parallel to the layer being charged.

It will be obvious to those skilled in the art that many modifications may be made Within the scope of the present invention without departing from the spirit there of, and the invention includes all such modifications.

What is claimed is:

1. An apparatus for electrostatically charging a photoconductive layer comprising at least one discharge electrode formed by a plurality of electrically connected spaced needles mounted substantially perpendicularly to and extending across a path traversed by the layer, at least one auxiliary electrode adjacent to and spaced laterally from the discharge electrode, at least one counterelectrode positioned on the other side of the path, the distance between the discharge electrode and the path being greater than the distance between the discharge electrode and the auxiliary electrode and the latter distance being greater than the distance between adjacent needles, means connecting the discharge electrode with one pole of a source of DC. voltage, and means connecting the other pole of the source to the auxiliary electrode and the counter-electrode.

2. An apparatus according to claim 1 in which the auxiliary electrode is mounted substantially perpendicularly to the path traversed by the layer.

3. An apparatus according to claim 1 in which two auxiliary electrodes are mounted on opposite sides of the discharge electrode.

4. An apparatus according to claim 1 in which the auxiliary electrode comprises a strip extending across the path traversed by the layer and having an edge facing the path.

5. An apparatus according to claim 1 in which the auxiliary electrode is a rotatable roll.

6. An apparatus according to claim 1 in which the counter-electrode is a rotatable roll.

7. An apparatus according to claim 5 in which a feed roll is mounted on the opposite side of the path from the auxilary electrode roll.

8. An appartus according to claim 3 in which the auxiliary electrodes are rotatable rolls and feed rolls coacting therewith are mounted on the opposite side of the pat-h.

References Cited by the Examiner UNITED STATES PATENTS 2,641,025 6/1953 Busby 3173 X 2,692,948 10/1954 Lion 250- 2,922,883 1/1960 Giaimo 317-262 X 2,935,418 5/1960 Berthold et al. 317-262 3,147,415 9/1964 Oli-phant 317-262 X 3,196,765 7/1965 Walkup 1.7 X 3,233,156 5/1966 Jarvis et al 317262 FOREIGN PATENTS 205,557 8/1956 Austria. 1,082,400 5/ 1960 Germany.

MILTON O. HIRSHFIELD, Primary Examiner.

SAMUEL BERNSTEIN, Examiner.

MAX L. LEVY, I. A. SILVERMAN,

Assistant Examiners. 

1. AN APPARATUS FOR ELECTROSTATICALLY CHARGING A PHOTOCONDUCTIVE LAYER COMPRISING AT LEAST ONE DISCHARGE ELECTRODE FORMED BY A PLURALITY OF ELECTRICALLY CONNECTED SPACED NEEDLES MOUNTED SUBSTANTIALLY PERPENDICULARLY TO AND EXTENDING ACROSS A PATH TRAVERSED BY THE LAYER, AT LEAST ONE AUXILIARY ELECTRODE ADJACENT TO AND SPACED LATERALLY FROM THE DISCHARGE ELECTRODE, AT LEAST ONE COUNTERELECTRODE POSITIONED ON THE OTHER SIDE OF THE PATH, THE DISTANCE BETWEEN THE DISCHARGE ELECTRODE AND THE PATH BEING GREATER THAN THE DISTANCE BETWEEN THE DISCHARGE ELECTRODE AND THE AUXILIARY ELECTRODE AND THE LATTER DISTANCE BEING GREATER THAN THE DISTANCE BETWEEN ADJACENT 